The World Wide Fund for Nature (WWF) Norway chief executive, Nina Jensen, said she was "cautious about drawing conclusions from one mild winter into specific changes in nature", but there were signals that changes were happening.

"We are definitely seeing plants like bluebells flowering that wouldn't come out until spring, and birds singing that wouldn't normally be at this time of year. There are quite obvious changes in the growth season, plant growth and migratory bird routes and timing. The flip side of this warmer winter is that we will also have an increasing threat of harmful introduced organisms, such as the wild boar or ticks that thrive in warmer temperatures."

Pål Hermansen, a wildlife photographer based in Oslo, said: "It's the smaller things where you see it most, especially butterflies and other insects. The combination of 'proper winters' with lots of snow, alternating with winters like this one, makes everything very unstable. In the 30 years I've been working we've seen butterfly populations reduce by 80-90%. We're now seeing mosquitos and ticks during the winter, which is unheard of. Ticks are spreading much further north than they ever were before."

The thought of dengue in Norway and Lyme disease in Sweden is hard to absorb.

The World Wide Fund for Nature (WWF) Norway chief executive, Nina Jensen, said she was "cautious about drawing conclusions from one mild winter into specific changes in nature", but there were signals that changes were happening.

"We are definitely seeing plants like bluebells flowering that wouldn't come out until spring, and birds singing that wouldn't normally be at this time of year. There are quite obvious changes in the growth season, plant growth and migratory bird routes and timing. The flip side of this warmer winter is that we will also have an increasing threat of harmful introduced organisms, such as the wild boar or ticks that thrive in warmer temperatures."

Pål Hermansen, a wildlife photographer based in Oslo, said: "It's the smaller things where you see it most, especially butterflies and other insects. The combination of 'proper winters' with lots of snow, alternating with winters like this one, makes everything very unstable. In the 30 years I've been working we've seen butterfly populations reduce by 80-90%. We're now seeing mosquitos and ticks during the winter, which is unheard of. Ticks are spreading much further north than they ever were before."

The thought of dengue in Norway and Lyme disease in Sweden is hard to absorb.